Redesign complete AVL Tree Implementation

[Alokzh]

This PR completely redesigns AVL Tree Data Structure repo with cleaner architecture and better performance

Changes:

• Refactored class hierarchy with abstract base node
• Enhanced balancing logic and rotation handling
• Added missing API methods (range queries, predecessor/successor)
• Improved validation and comprehensive test coverage

Testing

• Added Tests covering all functionality, rotations, and edge cases. All passing.